How to add CSS/Javascript version number in Magento 1

app/code/core(or local)/Mage/Page/Block/Html/Head.php

Change this:
$html .= $this->_prepareStaticAndSkinElements('
' . "\n",
    empty($items['js_css']) ? array() : $items['js_css'],
    empty($items['skin_css']) ? array() : $items['skin_css'],
    $shouldMergeCss ? array(Mage::getDesign(), 'getMergedCssUrl') : null
);

To this:
$html .= $this->_prepareStaticAndSkinElements('
' . "\n",
    empty($items['js_css']) ? array() : $items['js_css'],
    empty($items['skin_css']) ? array() : $items['skin_css'],
    $shouldMergeCss ? array(Mage::getDesign(), 'getMergedCssUrl') : null
);